There are many types of torque sensors for detecting a torque that are used in rotating shafts. Magnetostrictive torque sensors that have high precision and a comparatively simple configuration have recently been disclosed in JP-A-2001-133337, JP-A-2004-309184, JP-A-2004-333449, JP-A-2004-340744.
The magnetostrictive torque sensors disclosed in JP-A-2001-133337, JP-A-2004-309184, JP-A-2004-0333449, JP-A-2004-340744 are provided to electric power steering apparatuses for vehicles, and are used to detect a steering torque transferred from a steering wheel to a torque transmission shaft (rotating shaft). A magnetostrictive film is formed on an external peripheral surface of the torque transmission shaft. The magnetostrictive torque sensor makes it possible to detect a steering torque by using an electric coil and a magnetostriction sensor circuit to detect changes in magnetostriction that occur in the magnetostrictive film in accordance with the steering torque.
The steering torque generated by the steering wheel is transferred to the steered vehicle wheels via the torque transmission shaft, a rack and pinion mechanism, and a rack shaft. Thus, a magnetostrictive film is formed on the external peripheral surface of the torque transmission shaft, and a pinion (torque transmission portion) of the rack and pinion mechanism is formed at the end of the shaft.
There is a need for the ability to steer an automobile even when the engine has not been started up. In this state, the steering torque for steering the steered vehicle wheels is greater than during usual steering. The greater steering torque is transferred from the torque transmission shaft to the rack shaft via the rack and pinion mechanism. Therefore, the rack and pinion mechanism requires great mechanical strength. Specifically, the rack and pinion mechanism is subjected to various external forces originating from the reactive force of the road surface, as well as moderate external force caused by the steering of the driver. The rack and pinion mechanism requires mechanical strength that is sufficient to maintain the steering state despite these external forces.
The pinion of the rack and pinion mechanism requires sufficient strength that is needed to transmit great steering torque (steering torque that exceeds usual steering torque and corresponds to a greater load). Therefore, in many cases the pinion undergoes various surface treatments, such as carburization, high-frequency hardening, and other heat treatments, as well as shot peening, and the like.
However, performing heat treatments on the pinion causes the carbon components to diffuse in the surface of the torque transmission shaft that has the pinion. As a result, the surface of the torque transmission shaft is easily magnetized. Performing shot peening or another surface curing treatment on the pinion causes compressive stress to remain in the surface of the torque transmission shaft.
The magnetostrictive film formed on an external peripheral surface of the torque transmission shaft is commonly comprised of an Ni—Fe alloy film or another magnetostrictive plating material. This type of magnetostrictive plating material is highly susceptible to the effects of magnetism from the torque transmission shaft and to the effects of strain in the torque transmission shaft. The magnetostrictive film also receives the effects of external magnetism (for example, geomagnetism or noise from magnetism in the vehicle or the like) via the torque transmission shaft.
Thus, in cases in which the torque transmission shaft is provided with both a magnetostrictive film and a member that requires high strength, such as a pinion (torque transmission portion), there is room for improvement in increasing stability while maintaining a high level of sensitivity in the magnetostrictive properties of the magnetostrictive film. Increasing the stability of the magnetostrictive properties is related to the stability of the sensor signals generated by the magnetostrictive torque sensor.
In view of this, a technique is needed whereby a torque transmission shaft can be provided both with a magnetostrictive film and with a torque transmission portion by using appropriate processing, and whereby the sensitivity and stability of the magnetostrictive properties of the magnetostrictive film can be increased.